As used herein, a hard-metal is a material comprising grains of metal carbide such as tungsten carbide (WC) or titanium carbide (TiC), dispersed within a binder phase comprising a metal such as cobalt (Co), nickel (Ni) or metal alloy. The binder phase may be said to cement the grains together as a sintered compact, typically having negligible porosity. The most common hard-metal is Co-cemented WC.
Hard-metals are used in a wide variety of applications, particularly in applications where a tool needs to be resistant against wear and other mechanical degradation. Examples of such applications include tools for machining, cutting, drilling or otherwise mechanically forming or degrading a work-piece or other body. Hard-metal inserts are widely used for machining metals and abrasive materials, or as tools for mechanical picks for degrading pavements, asphalt or rock formations, or as inserts for drill bits for earth and rock boring in the oil and gas industry, as well as protective parts potentially subject high rates of mechanical wear in use, known as wear parts. Hard-metals used in these kinds of applications may be subjected to high impact loads, intensive wear, severe fatigue, high temperatures and strong thermal shocks in use, and are typically engineered to possess an outstanding combination of hardness and fracture toughness, as well as associated properties of high strength and abrasion resistance. Typically, abrasion resistance is positively correlated with hardness. Hard-metals are also used as supporting substrates for polycrystalline diamond (PCD) and polycrystalline cubic boron nitride (PCBN) elements, to which they are typically integrally bonded during a sintering step carried out at an ultra-high pressure, which is understood to mean a pressure greater than about 2 GPa.
The hardness and toughness of a type of hard-metal can be determined by selecting particularly the mean carbide grain size, the binder content and the carbon content within the binder. Unfortunately, hardness and toughness tend to be favoured by different sets of content and microstructure, and conventionally the one can only be increased at the expense of the other.
In a publication entitled “Measurement of residual thermal stress in WC—Co by neutron diffraction” (Mari, D. and B. Clausen in The International Journal of Refractory and Hard Materials, volume 27 of 2009, pages 282 to 287) it is disclosed that the residual stress in the binder phase of the surface region of conventional hard materials may be up to 1,850 MPa or even to 2,000 MPa.
The content of Co binder and C within Co-cemented WC hard-metals can be determined by selection of the starting powders. The C content within the binder can be decreased by adding W metal or increased by adding carbon black. The mean size of the WC grains can be controlled by introducing a known grain growth inhibitor or by limiting the amount of carbon present, which directly influences the amount of W dissolved in the binder. A disadvantage of the first approach may be that grain growth inhibitors reduce the toughness of the hard-metal. A disadvantage of the second approach may be that carbon content must be as low as possible within the two-phase range of WC—Co, which is technically difficult, since low carbon content may result in the formation of brittle eta-phases, which would also reduce the toughness.
United Kingdom patent number GB1506915 discloses the discovery that it is possible to make a cemented carbide body containing a binder metal and one or more hard metal carbides, with a thin wear-resistant surface layer by treating the body with carbon monoxide. It further discloses a cutting tool comprising such a body, wherein the concentration of carbide is greater than that within the remainder of the body and decreasing from the exterior of said layer towards the interior.
Hard-metal bodies having graded structure and properties within a region may be made by introducing a grain growth inhibitor locally, thereby locally limiting the mean size of the carbide grains. For example, U.S. Pat. No. 5,623,723 discloses a method for making a graded Co-cemented WC hard-metal by heat treating a green body (i.e. a body comprising the constituents of a hard-metal, but not yet sintered) in contact with a source of grain growth inhibitor. European patent number 1 548 136 discloses a cemented carbide wherein the grain size of the carbide within a surface portion is smaller than that within an interior portion, the binder content being lower within the surface portion than in the interior portion.
Normally, the microstructure of conventional WC—Co hard-metals must be two-phase comprising only the WC phase and the binder phase. However, as a result of decarburisation, additional phases, which are generally designated in the literature as “eta-phases” may form.
U.S. Pat. No. 4,820,482 discloses a method for making a body having varying binder phase content and substantially no eta-phase by carburising a WC-containing body having sub-stoichiometric carbon content. As a result of the carburising treatment, a body is obtained comprising a low content of binder phase in the surface zone (possibly along with small amounts of free graphite) and a high content of binder phase in the centre.
U.S. Pat. Nos. 4,743,515 and 5,856,626 disclose a graded Co-cemented WC hard-metal comprising at least two regions, a surface region and a core region, wherein the surface region is substantially devoid of eta-phase and the core region contains eta-phase. United States patent application publication number 20080240879 discusses that the method in U.S. Pat. No. 4,743,515 has the disadvantage that it results in a binder phase gradient that is rich in cobalt over one or two millimeters, while the core of the hard-metal remains fragile because it is constituted by the eta-phase and can easily crack during repeated impacts.
U.S. Pat. No. 5,066,553 discloses a surface-coated tool member of WC-based cemented carbide which has a hard coating formed on a substrate. The cobalt content of the substrate in a surface portion of depth about 2 microns is less than that at a depth of about 100 microns by at least 10 percent.
United States patent application publication number 20050147850 discloses a cemented carbide body comprising WC and a Co or Ni binder phase with a nominal binder phase content of 4 to 25 weight percent, and a surface portion and an interior portion. The surface portion has a binder phase content less than 0.9 of the binder phase content in the interior portion.
Japan patent number 02209448A discloses an ultra-hard alloy comprising WC and a bonding phase of iron-group metal, the content of a bonding phase in a surface region being less than that in the interior, resulting in residual stress in the surface region. Segregation of bonding phase into the interior may be achieved by a method including repeated carburisation and decarburisation of a packed powder pre-form.